In conventional "CODECs" the transmission of moving images at very low data rates requires, in addition to employing a device for compression of the transmitted data, developing techniques for limiting the spatial and time resolution of the image signal. The limitation of the spatial resolution is obtained by sub-sampling the image in the vertical and horizontal directions. In this field it is known to use the CIF and QCIF formats, which are respectively the abbreviations of 37 Common Intermediate Format" and "Quarter of Common Intermediate Format". In the CIF format the number of points per line in luminance is equal to 352, the number of points per line in chrominance is respectively 176 for the Dr and Db components and the number of lines per image is 288 for the luminance signals and 144 respectively for the Dr and Db chrominance signals. In the QCIF format the number of points per line in luminance is 176, the number of points per line in chrominance is respectively 88 for the Dr and Db signals and the line number per image is 144 for the luminance signals and 72 respectively for the Dr and Db chrominance signals.
The limitation of the time resolution takes place quite simply by reducing the number of images transmitted with respect to the number of images delivered by the source. In practice, in the presence of an image source to the CIF standard at 25 Hz, ie. 25 images per second, the reduction may go as far as making it possible to code and transmit images at a frequency of 1.25 Hz.
The currently known coders and decoders generally conform to recommendation H261 version 11 1989 of the "CCITT specialist group on coding for visual telephony". In this context they are very well suited for data-rate applications which may vary by integer multiples of 64 Kbit/s. Below that it is difficult to produce coders which have both good spatial resolution and good time resolution. However, the transmission of images at very low data rate (32 Kbit/s, 16 Kbit/s, etc.) compels sub-sampling of the image signal to be carried out both in the spatial domain and in the time domain. Thus, the choice may be made to transmit images at 5 Hz in order to obtain spatial resolutions reduced to the QCIF format and at 1.25 Hz to obtain spatial resolution reduced to the CIF format, but in these conditions the CIF 1.25 Hz standard is rather intended for slowly moving images, while the other standard is more appropriate for scenes with a great deal of movement.